The present invention relates to an exhaust emission control system for an internal combustion engine having a nitrogen oxide (NOx) removing device in an exhaust system, and more particularly to an exhaust emission control system capable of performing a regeneration process for the NOx removing device in the case that it is deteriorated by sulfur poisoning.
A conventional exhaust emission control system known in the art includes a NOx removing device in an exhaust system of an internal combustion engine. In this exhaust emission control system, NOx is absorbed by the NOx removing device in a lean operation where the air-fuel ratio is set in a lean region with respect to a stoichiometric ratio, and NOx absorbed by the NOx removing device is reduced to be discharged by suitably executing enrichment of the air-fuel ratio. The NOx absorption capacity of the NOx removing device decreases from sulphur poisoning because it also absorbs oxides of sulfur contained in the fuel. To cope with this problem, there has been proposed an exhaust emission control system having a SOx catalyst capable of absorbing sulfur oxide (SOx) provided upstream of the NOx removing device (e.g., Japanese Patent Laid-open No. 11-247650).
In the exhaust emission control system described in this publication, the amount of sulfur oxide absorbed by the SOx catalyst is estimated, and when the estimated amount of sulfur oxide reaches a set value, a regeneration process for removing the absorbed sulfur oxide is executed. To estimate the amount of absorbed sulfur oxide, a counter is provided and an addition value per unit time of the counter is set according to a rotational speed and an intake pressure of the engine. By incrementing the counter with the addition value, the amount of sulfur oxide absorbed by the SOx catalyst is estimated.
In the case that no SOx catalyst is provided in the exhaust system, sulfur oxide is absorbed in the NOx removing device. Accordingly, it is necessary to execute the regeneration process such that the air-fuel ratio is enriched in the condition where the temperature of the NOx removing device is high, thereby discharging the absorbed sulfur oxide. In this case, the amount of sulfur oxide absorbed in the NOx removing device must first be estimated. However, if the technique described in the above publication is applied, the following problem arises.
Although frequently executing the lean operation where the air-fuel ratio of an air-fuel mixture to be supplied to the engine is set in a lean region with respect to the stoichiometric ratio, there is also a period of performing a stoichiometric operation where the air-fuel ratio is set to the stoichiometric ratio, or a rich operation where the air-fuel ratio is set in a rich region with respect to the stoichiometric ratio. When the temperature of the NOx removing device becomes high during the stoichiometric operation or the rich operation, the sulfur oxide absorbed in the NOx removing device are discharged. However, such discharge of the sulfur oxide during the stoichiometric operation or the rich operation is not considered in the conventional system described in the above publication, so that the estimation of the amount of sulfur oxide absorbed in the NOx removing device becomes inaccurate. This causes a problem in that the timing of execution of the regeneration process may deviate from the optimum timing.